This invention relates to dewpoint meters, for measuring the dewpoint of a gas, of the kind which includes a sensor having a mirror or other surface exposed to the gas, an optical detector or other means which produces an electric signal upon the detection of condensation on the surface, an electrically operated device for controlling the temperature of the surface, the electrically operated device being controlled in dependence upon the signals produced by the detector or other means to maintain the temperature of the surface at that at which condensation just takes place upon it, and a thermometer to indicate the temperature of the surface at which condensation occurs, this being the dewpoint of the gas. Such a dewpoint meter is disclosed, for example, in U.S. Pat. No. 3,112,648.